With the rapid development of urban construction in China, there are more and more underground space development projects, such as subway projects, underground passages, underground commercial streets, etc. The construction methods of underground engineering are mainly open-cut sequential excavation method, cover-cut sequential (reverse) construction method and subsurface excavation method. The subsurface excavation method is further divided into shallow buried excavation method, shield method, pipe jacking method and NTR method, etc. In urban projects with good geological conditions, the shallow buried excavation method is widely used, but tunnel specifications and textbooks in different industries provide different methods for the calculation and design of initial support, making it difficult for designers to choose.

In the calculation of initial support, whether using the load-structure model or the stratum-structure model, it is difficult to determine the load sharing ratio or load release rate after the initial support is installed.

Generally, when using the shallow buried excavation method, the rough tunnel is excavated first and concrete is sprayed initially in time, then steel frames and steel mesh are erected, and then concrete is sprayed to the design thickness. In the calculation, how to determine the steel frame, how much stratum load the steel frame will bear, or whether it is sufficient to calculate the combined bearing of the steel frame and sprayed concrete, or to use only the engineering analogy method for design.

I am only raising such a question here and providing the calculation process of a project I once participated in designing, hoping to attract the attention and discussion of tunnel design and research personnel. To provide a unified model for the design and calculation of initial support, which can ensure the reliability and economy of the design and provide convenience for tunnel designers.

The figure below shows the force diagram of the steel frame in the initial support during CRD excavation.